(1) Field of the Invention
This invention relates to a photomask, a photomask fabrication method, and a semiconductor device fabrication method and, more particularly, to a photomask having the function of shifting the phase of exposure light, a method for fabricating such a photomask, and a method for fabricating a semiconductor device by the use of such a photomask.
(2) Description of the Related Art
In recent years the formation of very fine patterns has been needed for manufacturing large-scale integrated circuits (LSIs). Accordingly, photomasks (phase shift masks) having the function of heightening contrast by shifting the phase of exposure light are used in exposure processes in which a fine circuit pattern is transferred onto a semiconductor substrate (wafer). In particular, phase shift masks of a halftone type are widely used at present for manufacturing devices because of, for example, easiness of fabrication.
FIGS. 12(A) and 12(B) show the structure of a conventional typical phase shift mask of a halftone type. FIG. 12(A) is a top view of a conventional typical phase shift mask of a halftone type. FIG. 12(B) is a sectional view taken along the line B-B of FIG. 12(A).
A conventional phase shift mask 200 of a halftone type comprises a semitransparent halftone phase shift layer (phase shift layer) 202 and a light shielding layer 203 which are formed in order on a transparent substrate (synthetic silica substrate, for example) 201. In a chip region 210 where circuit patterns 210a transferred onto a semiconductor chip (chip) are to be formed, the light shielding layer 203 is removed and the phase shift layer 202 in which openings corresponding to the circuit patterns 210a to be formed are made is exposed. In an exposure process, there is no change in the phase of exposure light which passes through areas in the chip region 210 where the transparent substrate 201 is exposed, and the phase of exposure light which passes through areas in the chip region 210 where the phase shift layer 202 is exposed is inverted.
On the other hand, the light shielding layer 203 is left in a region outside the chip region 210 so that when the circuit patterns are transferred onto a wafer, multiplex exposure will not be performed on adjacent chips (see, for example, Japanese Unexamined Patent Publication No. 2002-182367, paragraphs [0022]-[0027]and FIG. 1). Various patterns are also formed in this region. For example, a monitor pattern 220 for measuring the accuracy of the position of the circuit patterns 210a, a target pattern 221 for performing alignment at overlay drawing time in the process for fabricating the phase shift mask 200 with a mask drawing apparatus, a pattern 222 for measuring alignment accuracy used at overlay drawing time, a fiducial pattern 223 for adjusting the relative positions of the phase shift mask 200 and an exposure system (stepper or scanner), a pattern 224 for performing alignment at the time of inspecting the circuit patterns 210a for a defect (or for performing alignment at the time of measuring the line width of the circuit patterns 210a), a bar code pattern 225 for mask identification, and a numbering pattern 226 for mask identification are formed.
Next, a method for fabricating a conventional phase shift mask will be described.
FIGS. 13(A), 13(B), 13(C), and 13(D) and FIGS. 14(A), 14(B), and 14(C) are sectional views each showing one step included in a method for fabricating a conventional phase shift mask.
First, a phase shift layer 202a, a light shielding layer 203a, and a resist layer 204a are formed on a transparent substrate 201 in that order (FIG. 13(A)).
Exposure and development are then performed on the resist layer 204a to form a desired resist pattern 204b (FIG. 13(B)). After that, the light shielding layer 203a and the phase shift layer 202a are etched in order to form a light shielding layer 203b and a phase shift layer 202 in each of which openings corresponding to the resist pattern 204b are made (FIG. 13(C)).
The resist pattern 204b is removed. and a resist layer 205a is formed again (FIG. 13(D)).
After the resist layer 205a is formed, exposure and development are performed on the resist layer 205a to form a resist pattern 205b in which an opening is made in a chip region 210 (FIG. 14(A)).
The light shielding layer 203b which is exposed in the chip region 210 is etched to form a light shielding layer 203 in which an opening is made in the chip region 210 (FIG. 14(B)). Lastly, the resist pattern 205b is removed and the phase shift mask 200 is completed (FIG. 14(C)).
When the circuit patterns are transferred onto a wafer by using the conventional phase shift mask, blinds of an exposure system prevent light from leaking out of the chip region. However, a pattern outside the chip region is transferred onto the wafer by the influence of a flare produced by reflection by a lighting system and lenses of the exposure system and the phase shift mask.
If the conventional phase shift mask of a halftone type is used, a resist layer in the entire chip region is slightly exposed. Accordingly, if the influence of a flare is added, an abnormality tends to occur in circuit patterns formed on the wafer.
The patterns, such as the monitor pattern, located outside the chip region are necessary for fabricating the phase shift mask, except the fiducial pattern for adjusting the relative positions of the phase shift mask and the exposure system. Therefore, these patterns must characteristically be located. as close. to the chip region as possible (the validity of a guarantee deteriorates with an increase in the distance from the chip region). However, locating these patterns close to the chip region increases the possibility that they are transferred onto the wafer by the influence of a flare.